Psittacosis is a zoonotic infection caused by Chlamydia psittaci, an obligate intracellular gram-negative bacterium that causes avian chlamydiosis in birds and systemic disease in humans [1][5]. It is classified among the most common atypical pneumonias, which are systemic infectious diseases with a pulmonary component that may be differentiated from typical community-acquired pneumonias by characteristic patterns of extrapulmonary organ involvement [5].
Disease overview
Bacterial Evidence-backed information EN 5/7 applicable sections · ZH 5/7 applicable sections Updated Oct 23, 2026
Chlamydia psittaci is an obligate intracellular gram-negative bacterium with a biphasic developmental cycle that causes avian chlamydiosis in birds and psittacosis in humans [1]. It is a zoonotic pathogen with a wide range of avian hosts, and human infection occurs primarily through exposure to infected birds [1][2]. Person-to-person transmission has been confirmed in the last decade, though bird contact remains the dominant route [1][3]. The disease manifests as an atypical pneumonia with systemic features, ranging from asymptomatic infection to severe disease [2][4].
Read the full clinical and epidemiological profile 6
Psittacosis presents with a typical clinical picture of atypical pneumonia, including approximately one week of fever, headache, myalgias, and nonproductive cough [4]. Pneumonia is the most common manifestation, though all organ systems may be involved as these atypical agents often cause extrapulmonary manifestations [5][4]. The infection is associated with a spectrum of disease severity ranging from asymptomatic infection to severe atypical pneumonia and systemic disease [2].
Current disease surveys indicate that the highest incidences of human psittacosis exist in Australia, Germany, and the United Kingdom [3]. In Belgium, reported positive laboratory results increased slowly since 2010, nearly doubling in 2017 compared to the two previous years [2]. The number of psittacosis cases is probably highly underestimated in Belgium and other countries due to underdiagnosis and underreporting; the mandatory notification system captured only 24% of all reported positive laboratory results over a three-year period [2].
The primary transmission route is through exposure to infected birds, including both horizontal transmission between birds and cross-species transmission to humans [1]. Human-to-human transmission has recently been confirmed, though bird contact remains the dominant route [1][3]. The pathogen's wide range of avian hosts facilitates ongoing transmission to humans [1].
C. psittaci is primarily transmitted horizontally among birds, and although cross-species transmission to humans occurs, prevention is framed within a One Health approach that integrates avian chlamydiosis and human psittacosis [1]. A basic understanding of the pathogen's genotype, host–pathogen interactions, transmission, epidemiology, detection, and clinical management supports targeted control strategies across avian and human populations [1].
- 1 Wang J et al. Chlamydia psittaci: A zoonotic pathogen causing avian chlamydiosis and psittacosis. Virulence. 2024 Dec. PMID: 39541409. doi: 10.1080/21505594.2024.2428411. PubMed: https://pubmed.ncbi.nlm.nih.gov/39541409/
- 2 Rybarczyk J et al. Human psittacosis: a review with emphasis on surveillance in Belgium. Acta Clin Belg. 2020 Feb. PMID: 30882289. doi: 10.1080/17843286.2019.1590889. PubMed: https://pubmed.ncbi.nlm.nih.gov/30882289/
- 3 Dembek ZF et al. Psittacosis: An Underappreciated and Often Undiagnosed Disease. Pathogens. 2023 Sep 15. PMID: 37764973. doi: 10.3390/pathogens12091165. PubMed: https://pubmed.ncbi.nlm.nih.gov/37764973/
- 4 Stewardson AJ et al. Psittacosis. Infect Dis Clin North Am. 2010 Mar. PMID: 20171542. doi: 10.1016/j.idc.2009.10.003. PubMed: https://pubmed.ncbi.nlm.nih.gov/20171542/
- 5 Cunha BA et al. The atypical pneumonias: clinical diagnosis and importance. Clin Microbiol Infect. 2006 May. PMID: 16669925. doi: 10.1111/j.1469-0691.2006.01393.x. PubMed: https://pubmed.ncbi.nlm.nih.gov/16669925/
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